This invention relates to a color cathode ray tube, more particularly to the supporting structure for the shadow mask within the tube.
In a color cathode ray tube having a shadow mask, the shadow mask conventionally is secured to supporting studs in the glass envelope of the tube by supporting members. The supporting members usually comprise bimetal segments which compensate for the thermal expansion of the shadow mask during electron beam bombardment. In order for the electron beam to follow the right path toward the phosphor screen of the tube, the shadow mask must be displaced slightly toward the phosphor screen to compensate for thermal expansion of the shadow mask. The bi-metal segments, which conventionally consist of two metal layers secured on each other, displace the shadow mask toward the phosphor screen as described in U.S. Pat. No. 3,330,980 issued to Shrader on July 11, 1967. Each of the bi-metal segments is formed in two layers, one of which is a metal plate having a uniformly low coefficient of thermal expansion and the other of which is a metal plate having a high coefficient of thermal expansion. These bi-metal segments are carefully positioned to result in movement of the shadow mask toward the phosphor screen upon thermal expansion. However, the cost of manufacturing bi-metal segments is very high because these bi-metal segments must meet precise specifications to result in uniform and consistent operation.
Various improvements have been made in the bi-metal segments which support the shadow mask. For example, in U.S. Pat. No. 3,573,527 issued to Hafkenscheid et al on Apr. 6, 1971, a lateral bi-metal supporting member is shown having bi-metal segments connected together on the narrow side. However, welding the different metal segments along their narrow sides is still difficult and it is difficult to select the proper combination of these metals. Furthermore, because the metal segments are very thin and are welded together by seam welding, the bi-metal supporting member is not suitable to support a heavy shadow mask within a large size color cathode ray tube.
Similarly, other attempts to improve the structure of the bi-metal supporting members have not overcome the above mentioned disadvantages. In U.S. Pat. No. 3,823,336 issued to Nakamura et al on July 9, 1974, a pair of parallel bi-metal strips are shown to support the shadow mask. Although these strips will support a heavy shadow mask, these strips are similar to the bi-metal segments in the Shrader patent and suffer the same disadvantage of high manufacturing cost. Finally, in U.S. Pat. Nos. 3,399,319 and 3,487,251 issued to Schwartz et al and Barten et al, respectively, parallel strips having the same coefficient of thermal expansion are used to support the shadow mask. While these parallel strips overcome some of the disadvantages of manufacturing discussed above, they do not provide effective and precise control of the movement of the shadow mask during thermal expansion.